The term “stent” is intended to indicate devices intended for endoluminal application (e.g. within a blood vessel), normally fitted via catheterization, with subsequent deployment in situ so as to provide a local supporting effect for the lumen. Stents are extensively disclosed in the patent literature, for example co-pending, commonly assigned EP 0 806 190, EP 0 850 604, EP 0 857 470, EP 0 875 215, EP 0 895 759, EP 0 895 760, EP 1 080 738, EP 1 088 528, and EP 1 103 234.
In particular, the present invention falls within the line of research aimed at developing solutions that enable the stent to be used as a vehicle for delivery of active or activatable agents of various nature (e.g., pharmacological agents, radioactive agents, etc.) designed, for example, to inhibit restenosis. Solutions of this sort are described, in the framework of the documents cited above, in EP 0 850 604, EP 1 080 738, and EP 1 103 234.
In particular, EP 0 850 604 describes the possibility of providing, on the surface of a stent, in particular on its outer surface, a sculpturing having the function of increasing the surface area of the stent in such a way as to create undercuts and/or, in general, a surface roughness in order to facilitate application of coatings of active or activatable agents. The sculpturing, consisting for instance of microspheres, may also increase adhesion of the stent to the wall of the vessel being treated.
The most recent clinical developments have revealed that limiting the intervention to coating the surface of the stent with active or activatable agents may encounter various difficulties and/or problems.
First, the amount of agent with which the stent is coated may in actual fact prove insufficient, particularly when the aim is to obtain a release, and hence an action, that is prolonged in time. Further, in applications of vascular angioplasty, the surfaces of the stent, and in particular the inner surface, are subjected to an action of flushing by the blood flow.
Second, for the reason just mentioned, it is desirable that the active or activatable agent should be carried and released prevalently, if not exclusively, on the outer surface of the stent, and not on its inner surface. This is particularly true in the case where the agent applied on the stent is designed to perform an antagonistic function in regard to restenosis. In these situations, the function of the active agent, which is aimed at acting on the outer surface of the stent facing the wall of the vessel undergoing treatment may have unfavorable effects in areas corresponding to the inner surface; for example, the phenomena of neointima formation on the inner surface of the stent, which are considered to be beneficial in the phases subsequent to the implant phase, may be hindered.
In the above general framework, it would be desirable to have available stents that include reservoirs of active or activatable agents, possibly different from one another, available in sufficient quantities to achieve a beneficial effect that may be prolonged over time. Further, it would be desirable to have stents capable of carrying available agents that are different from one another, located selectively in different positions along the stent, with the additional possibility of selectively varying the dosages in a localized way, for instance achieving dosages that are differentiated in the various regions of the stent.
On the other hand, it should not be forgotten that a stent is always configured as a structural element, in the sense that, once placed in the implantation site and brought into its radially expanded condition, the stent must be able to maintain the perviousness of the treated vessel without being subject to appreciable phenomena of contraction or collapse resulting from the radial compressive loads applied on the stent by the walls of the treated vessel.
This explains why the known solutions, aimed at creating on the surface of the stent irregularities such as might contain or enable anchorage of coatings and/or agents of various nature, have so far involved only quite a contained portion of the cross section of the parts (radially expandable annular elements, longitudinal elements of connection) of which the stent is normally composed.